Dynamoelectric machines are subject to a number of failures, one source of trouble being the bearings which journal the shaft and rotor for rotation. Rollable element bearings, such as ball bearings, are generally quite reliable, and it has been found that the few failures which occur generally occur because a small particle of metal or other abrasive material somehow is introduced into the ball bearing. Once a small metal chip is knocked out of either the ball or the inner or outer race, this rather quickly causes progressive failure, with additional chips being rapidly produced to grind all of the balls or races and provide complete failure of the bearing.
In some applications, such as a desert location of an oil pipeline motor-driven pump, it is extremely difficult to keep particles of sand from somehow getting into the lubricant grease of the bearing. Even though the motor is a totally enclosed type, foreign matter may be introduced at the time of regreasing since, under such difficult environmental conditions, contaminating particles may get into the grease before it is introduced into the motor or the grease gun or may get on the grease nipple.
A number of motors, such as vertical shaft motors, are mounted to drive crude oil pumps, and such pumps have a seal to prevent leakage of the crude oil being pumped. However, upon failure of a bearing in the motor, the shaft may drop, due to gravity, and even a small drop of as little as 0.030 inch may rupture or cause failure of the seal in the pump. This could permit the escape of volatile fluid, which, if it should hit the hot motor, could cause a fire. If the pump and motor are at an unattended location, such fire could rage uncontrolled for many hours before the oil flow in the pipeline was stopped.
For use in these conditions or other similar difficult environmental conditions, motor customers have, on many occasions, requested a motor be furnished which eliminated or minimized the possibility of such hazard. The prior art method of counteracting such possibility was to either specify an extra high thrust bearing in the vertical shaft motor or in some way to specify a motor with bearings which were over-designed for the expected load. The problem was therefore met by trying to design very conservatively for any and all expected load conditions, and even extremely severe load conditions. The difficulty with such an attempted solution was that even with oversized bearings or extra high thrust bearings, they still could be ruined by the introduction of foreign matter.